Toothed belts having tooth elements and a surface rubber layer that are formed of a rubber mix containing chloroprene as the main rubber component, a fiber core adhesively treated with a resorcin-formalin-latex (RFL) and a tooth covering fabric material are known. The core can be a glass fiber cord or an aramid fiber cord made from glass or aramid filaments. Crimped nylon yarns can be used as fillings in the longitudinal direction of the belt. The belts can contain 10 to 25 cords with a pitch of 0.8 to 1.5 millimeters (mm). Conventionally, the teeth are covered with a fabric material made of 6,6 nylon. These belts can be used in automobiles.
The ambient temperature conditions in which automotive toothed belts of an overhead cam drive are used have recently become more and more demanding because engine compartments have become more compact and because of the adoption of the forced fuel system, i.e., fuel injection. These conditions have increased the ambient temperature within the engine compartment to which the belt is exposed. Conventional toothed belts that use chloroprene often experience cracking and/or breaking off of the rubber surface or tooth or deterioration of the cores due to flexing at high temperatures. The belt often breaks under these conditions.
Due to the requirements for greater surface durability of toothed belts in general and automotive toothed belts in particular, either hydrogenated nitrile rubber (HNBR) produced by hydrogenating acrylonitrile-butadiene copolymer or chlorosulfonated polyethylene (CSM) is used as the main rubber. Use of such a rubber is disclosed in, for example, Japanese Patent Application Laid-Open No. 62-159827.
Cords are conventionally treated with an RFL liquid in which the latex component is an emulsified latex having a vinylpyridine-styrene-butadiene terpolymer as a principal ingredient with which a copolymer is blended. Representative copolymers include butadiene-styrene copolymer and acrylonitrile-butadiene copolymer. However, the glass fiber cords and aramid fiber cords treated with this RFL latex experience hardening of the RFL liquid deposited thereon during vulcanization because, as it is presently theorized, radicals produced during the decomposition of the organic peroxides cause excessive crosslinking. The hardening results in a loss of flexibility. When the belt is bent in use, filament breakage often occurs, which results in premature belt breaking and a decrease in tensile strength of the belt. In particular, glass filaments, which are less flexible than aramid fibers are more liable to fail.
The rubber composition in the backing member and/or tooth elements, the adhesive bond of the glass or aramid fiber cord and the tooth covering fabric material are all in close relationship and interact to affect the surface life of the belt. For example, in some experiments in which toothed belts were run in a high temperature environment, RFL liquid deposited on the glass or aramid fiber cord became heat-aged even when the rubber composition used for the backing member and/or tooth elements was heat resistance HNBR or CSM rubber. The cord and the rubber composition separated from each other because of the reduced bond strength caused by heat aging. Furthermore, the flex resistance of the belt was substantially lowered.
Belts under high load and tension conditions require suitable reinforcement from the covering fabric material. In a high temperature environment, the tooth root flank covering fabric that comes in contact with the pulley teeth can wear early which leads to cracking at the tooth flank and loss of the belt. Therefore, the fabric does not provide sufficient protection of the tooth.
The present invention is directed to solving these problems, and it is a primary object of the invention to provide a toothed belt which can retain good heat, flex and wear resistance during operation under high temperature conditions and which can maintain good serviceability for a long time.